Mary has 12 marbles 3/12 of the marbles are yellow and 2/12 of the marbles are blue. the rest of the marbles are green. how many marbles are green?

3 yellow marbles + 2 blue marbles + x green marbles = 12 marbles
x green marbles = 12 marbles - 5 marbles
x green marbles = 7 green marbles

Of you take 12/12 (total number of marbles) minus 3/12 (amount of yellow marbles out of total) that leaves 9/12 and if you subtract 2/12 (amount of blue marbles from total) from that it leaves you with 7/12 (the amount of green marbles from the total.
Yellow=3
Blue=2
Green=7
Total=12